[cfe-dev] mmintrin.h files and clang-cl

[Nico Weber]

Hi,

cl.exe supports using SSE intrinsics even if the compiler when targeting a
CPU that doesn't support them – for example, when using intrinsics from
tmmintrin.h, the intrinsics will expand to SSSE3 instructions, but the
compiler won't generate SSSE3 instructions for regular C code. This can be
used for having a few functions that use SSSE3 in a translation unit (those
that use the intrinsics) and then call these from other functions in the
same translation unit after checking processor flags for support.

clang (and gcc) don't support this. With clang, if you want to use
tmmintrin.h, you have to build with -mssse3, and then the compiler can
generate SSSE3 instructions for all C code. This is because the intrinsics
get compiled into builtins, which then are turned into llvm vector
instructions which then no longer remember that they were intrinsics at
some point – they aren't different from the llvm generated for regular code.

This causes some compatibility issues between clang-cl and cl – see for
example http://reviews.llvm.org/D4415

It just occurred to me that in MS mode, we could implement the SSE
intrinsics as built-in asm blocks when the targeted CPU doesn't support
them, like so:

  #ifndef __SSSE3___
  static __forceinline __m128i _mm_abs_epi8(__m128i __a) {
    _asm pabsb xmm0, xmmword ptr __a
  }
  #else
  static __inline__ __m128i __attribute__((__always_inline__, __nodebug__))
  _mm_abs_epi8(__m128i __a)
  {
      return (__m128i)__builtin_ia32_pabsb128((__v16qi)__a);
  }
  #endif


The downsides of this would be that clang couldn't reason about these and
would insert lots of unnecessary stores and loads, and code using
intrinsics is probably performance-sensitive – but it'd be more compatible.

Are there other downsides, or reasons why this can't work at all? (My
expectation is that we won't want to do this, and that I can then point at
this thread for why.)

Thanks,
Nico
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